Various optical multiplexer and/or demultiplexer structures have been described in the literature.
For example, reference may be made to the following documents:
[1] ECOC 96, "Extremely compact 1.31 .mu.m-1.55 .mu.m phased array duplexer on InP with -30 dB crosstalk over 100 nm", R. Mestric et al., which describes a duplexer having two wavelengths: 1.3 .mu.m and 1.55 .mu.m; and
[2] ECOC 96, "Compact low loss 8.times.10 GHz polarisation independent WDM receiver", C.A.M. Steenbergen et al., which describes an integrated demultiplexer having detectors, and proposes to use two waveguide structures to solve the problem of polarisation.
The (de)multiplexers that are generally used today are of the type having an array of waveguides, as shown in accompanying FIG. 1. Such a demultiplexer is made up of two plane optical surfaces 10 and 12 separated by an array 14 of waveguides. The difference in path length .DELTA.L between two consecutive waveguides 14 is constant, and it makes it possible to perform phase-shifting, and therefore demultiplexing. In FIG. 1, P1 represents the equiphase plane for an input signal while P2 represents the equiphase plane for a signal coming from one of the outlets.
Such known multiplexers/demultiplexers have already done good service. Unfortunately, they are not entirely satisfactory.
The main drawback with such known devices is their high degree of temperature dependency which is intrinsic to the material used.
The refractive index of the material varies as a function of temperature, and so the path-length difference between two consecutive waveguides changes with changing temperature, thereby causing the peaks to be offset relative to the output waveguides.
SiO.sub.2 has a coefficient of variation of refractive index as a function of temperature that is small (giving rise to an offset of about 1 nm per 100.degree. C.). Unfortunately, that material offers only limited possibilities as regards monolithic integration (integration on the same material) of devices such as lasers, optical amplifiers, or detectors.
Monolithic integration, which enables production costs to be reduced (compared with hybridizing on different materials), is possible on InP. Unfortunately, the refractive index of InP varies considerably with temperature (giving rise to an offset of about 1 nm per 10.degree. C.). The Publication [3] "Polarisation independent 8.times.8 waveguide grating multiplexer on InP", Electronics Letters, Jan. 21, 1993, vol. 29, No. 2, M. Zirngibl et al., gives a variation of 1.5 nm per 10.degree. C. That heavy dependency requires the temperature to be controlled by means external to the device, e.g. in the form a Peltier-effect element, which increases the cost of implementing the device.
Naturally, for a wide-line demultiplexer with a small number of lines, such as the 2-line duplexer in reference [1] ECOC 96 with lines of 100 nm, temperature does not disturb demultiplexing or disturbs it only slightly. However, for a multi-wavelength application, and lines of 0.65 nm, the temperature instability must be controlled.